1. Field of the Invention
The present invention relates to surface acoustic wave (SAW) filters that utilize leaky waves.
2. Description of the Related Art
Surface acoustic wave (SAW) filters are practically used as bandpass filters for use in radio frequency (RF) circuits of communication devices and other devices. To efficiently utilize frequency resources for wireless communication, various frequency bands are allocated as communication bands for mobile phones and other devices. Accordingly, SAW filters are designed and developed according to various frequency specifications.
Japanese Unexamined Patent Application Publication No. 2013-102418 discloses a SAW element applicable to a radio frequency band of 2 GHz or higher. Specifically, preferable ranges are set for parameters for interdigital transducer (IDT) electrodes disposed on a piezoelectric substrate such that the film thickness is 7.5% to 8.0% of λ (where λ denotes an IDT electrode pitch) and the line occupancy ratio (duty ratio) is 0.55 to 0.65. As the piezoelectric substrate, a 45°-to-46° Y—X lithium tantalate (LiTaO3) single crystal is used. This SAW element realizes low loss in the passband by utilizing leaky waves even if the frequency band is a radio frequency band of 2 GHz or higher.
To realize low loss in the passband, the film thickness and the line occupancy ratio, which are parameters for the IDT electrodes, are set for the SAW element disclosed in Japanese Unexamined Patent Application Publication No. 2013-102418 so that propagation loss decreases in a range from a resonant frequency to an anti-resonant frequency of resonators of the SAW element. In the case of a narrow band specification, such as the case where the passband width is approximately 2.3% of the center frequency, low loss is realized in the passband of SAW filters by decreasing propagation loss in a range from a resonance point to an anti-resonance point of the resonators as in Japanese Unexamined Patent Application Publication No. 2013-102418.
However, in the case of a wide band specification, such as the case where the passband width is 3% or greater in the bandwidth ratio (passband width/center frequency), the propagation performance of SAW filters over the entire passband is not determined solely by the propagation loss in the range from the resonance point to the anti-resonance point of the resonators. That is, as the passband widens, it becomes more difficult to realize low loss in SAW filters by solely decreasing the propagation loss in the range from the resonance point to the anti-resonance point of the resonators.